Force/torque sensors compatible with magnetic resonance imaging (MRI) are required to develop haptic interfaces for neuroscientific investigations and robotic tools for interventional MRI. In this paper, we analyze the mechanical structure of classical and MRI compatible sensors from literature and demonstrate the critical role of the mechanical design on the sensing performance. A simple and efficient torque sensor based on light intensity measurement over optical fibers is introduced, which allows to place the electronic components outside the scanner room. By using a self-guiding flexible structure and optimal mirror placement, the sensitivity to transverse torque is reduced to 0.03% of the desired output torque.